"Your" parents are going away and leaving you in the care of your grandmother. But when you go to meet her at the train station, you discover two identical women. One's an alien, one's a spy, and you need to find the truth and thwart an extraterrestrial invasion.
Secret Agent Grandma provides examples of:
- Afterlife Express: One bad ending has you boarding a train, and realizing too late that it is a train of ghosts.
- Easily Thwarted Alien Invasion: In one of the good endings, you and your friends manage to defeat the aliens by singing "Happy Birthday to You".
- Embarrassing Nickname: Your grandma always calls you "Cookie", which makes you cringe.
- Evil Old Folks: The titular alien grandma.
- Hollywood Acid: There's an ending where the "grandma" spills acid on "you", dissolving you instantly.
- Identical Stranger: What kicks off the plot of the story — you meeting your grandma in the train station, before seeing another identical grandma on an adjacent platform. The book gives you the choice of which grandma you should follow, leading to the two story branches.
- Me's a Crowd: In one of the more baffling — but still good — endings, you realized your grandma, and her double, turns out to be participants in a cloning experiment, and there are six other grandmas following just behind. You then spend the summer pampered like a prince by your army of grandmas.
- Never Mess with Granny: In the good ending where your grandma turns out to be a secret government agent investigating alien activity, when an alien (disguised as a train's conductor) tries attacking you and her, grandma quickly unleashes a cap of whoop-ass on the alien imposter.
- No Ending: At one point, you and your grandmother think that someone is lurking in wait for you inside the bathroom on a train, so you open the door — but someone rushes past you wanting to use the bathroom, and knocks you over, causing Grandma's spy gadget to land in the toilet bowl. You're told that this is the end, although it's not explained why, and this seemed more like a cliffhanger than an ending.
- Schrödinger's Gun: Your granny can turn out to be either a secret agent who investigates an alien plot, an alien impostor in human disguise, or a secret agent protecting a very important floppy disk... which turns out to contain all the cool video games from next year.
- Shout-Out: You can defeat the evil grandma's army of alien maggots by singing, much like the ending of Mars Attacks! released just a year ago.
- Skewed Priorities: The spy storyline have you finding out the contents of a powerful diskette your grandma to be entrusted with to be full of soon-to-be released video games. By doing so, you end up becoming so engrossed over the games until you fail to realize the government agents sneaking upon you. Whoops.
- Something about a Rose: One storyline involves mutant alien roses. They later reappear in You're Plant Food!
- Spot the Imposter: Happens twice. First you have to decide which of the two grandmas is the real one to determine which storyline you will take part in, and later in one of them you have to deal with the more traditional version of the trope with both grandmas warning you that the other is fake, and having to choose.
- This Loser Is You: In classic GYG manner, one of the bad endings have you choosing to stay on earth to observe extraterrestrial life from the comfort of your home, rather than going to space to meet the aliens your grandma told you about. The book then says you're really unsuited for Give Yourself Goosebumps, telling you to stick to nursery rhymes and come back when you stop being a wimp!
- Tomato Surprise: The alien route where your grandma turns out to be an alien imposter has a bad ending where you discover you are also an alien. What?
- Weaksauce Weakness: One of the scenarios ends with the alien eggs hatching, releasing an entire swarm of alien maggots which threatens to devour you and your friends. You have the option to turn on the radio, at which point you found out the aliens' weakness — music. You then defeat the aliens, firstly using the radio, then by singing.
